Free 500 Rule & NPF Calculator

The 500 rule is the classic formula for the maximum shutter time before stars streak: 500 divided by the full-frame-equivalent focal length. The maximum shutter time before stars visibly trail — computed three ways: the classic 500 rule, the stricter 300 rule for high-res sensors, and the NPF rule, which accounts for your camera's actual pixel size and aperture.

Your setup

Camera

Focal length (mm)

Aperture (f/)

Target declination (0° = celestial equator)

Stars near the celestial pole move slower — NPF relaxes by up to 2× at ±60°. LonelySpeck-style declination-aware calculation.

Max exposure times

NPF rule (accurate)

—

pixel-size aware

500 rule

—

classic, forgiving

300 rule

—

safer for high-res

600 rule

—

forgiving, film-era

🌌 How to use it: set your shutter to (or just under), open the aperture fully, then raise ISO until the histogram is ⅓–½ from the left. Check at 100% zoom: if stars are still elongated, shorten by 20%.

Worked example

A 20 mm lens on a Sony A1 II (full frame, 4.17 µm pixels) at f/2: the 500 rule says 500/20 = 25 s, but the NPF rule — which accounts for your real pixel size — says (35×2 + 30×4.17) / 20 ≈ 9.8 s. That's the difference between tack-sharp stars at 100% zoom and faint trails you only notice in post. Use the 500 rule as a quick mental check, but trust NPF on a modern high-resolution sensor; if you're shooting 135 mm+, even NPF gets aggressive and the calculator's pixel-scale number (arcseconds per pixel) tells you whether your rig can actually resolve stars at all.

Which rule should you use?

NPF rule — the accurate one. Formula: t = (35·N + 30·p) / f, where N is aperture, p is pixel size in µm, and f is the full-frame equivalent focal length. It accounts for both your sensor's pixel density and lens speed. Use this.
500 rule — t = 500 / (focal × crop). Fine for older, lower-resolution sensors; visibly trails on 24 MP+ cameras at 100% view.
300 rule — t = 300 / (focal × crop). A safer general choice for modern high-res sensors.

All three assume you're shooting away from the celestial poles; pointing near Polaris (or Sigma Octantis) lets you push times 2–3× longer before trails appear.

Related tools & guides

Astro Pixel Scale Calculator

Is your sampling matched to your optics and seeing?

Open →

Depth of Field for Astro

Focusing at infinity, the f-ratio trade-off, and stacking.

Read →

Exposure Equivalence

Balance ISO and shutter to hit the exact exposure.

Open →

🎒 Want this as a laminated field card? The ShutterMath Field Kit has all six, print-ready.

Star exposure FAQ

Why do stars trail at long exposures?

The Earth rotates ~15°/hour. A longer shutter moves each star across more pixels; when the movement exceeds about one pixel (or the pixel-size tolerance in NPF), the star becomes a streak instead of a point.

Does the 500 rule still work on my 45 MP camera?

Poorly — it was calibrated for 12–16 MP sensors. At 45 MP the pixels are ~4.4 µm and trails are visible well before 500/f. The NPF rule (or 300/f) is the right choice for modern sensors.

What if I want star trails on purpose?

Then ignore all rules — stack many long exposures (30 s–5 min each) and composite. The calculator's job is the opposite: keeping stars as points.